Conventionally, as a back-light device for a liquid crystal display, devices equipped with a cold-cathode tube as a light source are widely used. There are types of such devices called an edge light type and a direct type. The edge light type device is a back-light device which includes thin cold-cathode tube arranged at an end surface of a light guide plate, and light which has been incident from the end surface repeatedly reflects in the light guide plate, and then, is emitted to a main surface of the light guide plate. On the other hand, the direct type back-light device is that which includes a combination of a plurality of cold-cathode tubes arranged in parallel, a reflection plate provided behind the cold-cathode tube, and a light diffusion plate which serves as a light-emitting surface. In contrast to the edge light type device, in the direct type back-light device, the number of cold-cathode tubes to be used can be increased, and therefore, brightness on the light-emitting surface can be easily increased.
However, the direct type back-light device has a problem of poor brightness uniformity on the light-emitting surface. Particularly, periodical unevenness in brightness caused by high brightness at portions immediately above the cold-cathode tubes is a major problem. That is, when the brightness uniformity on the light-emitting surface of the back-light device is poor, unevenness in display occurs on a display screen of a liquid crystal display.
In the direct type back-light device, it is possible to improve the brightness uniformity by decreasing intervals between the cold-cathode tubes. However, such a solution results in an increased number of cold-cathode tubes, which causes a problem in that a structure of the back light becomes complicated and power consumption at the time of illumination increases. Alternatively, it is possible to improve the brightness uniformity by increasing a distance between the cold-cathode tubes and the light diffusion plate. In that case, however, the back-light device becomes thick, and thus, it is not possible to realize a thin liquid crystal display.
Conventionally, various measures to improve the brightness uniformity have been taken. For example, there has been proposed a method in which a light amount correction pattern consisting of a strip or a dot pattern is printed on the light diffusion plate to decrease a light amount emitted immediately above the cold-cathode tubes to relatively increase a light amount emitted between the cold-cathode tubes (illustrated in FIG. 6 of Patent Document 1), and a method in which a wavy reflection plate is used to collect reflection light from the reflection plate in a region which corresponds to a midpoint between the cold-cathode tubes (Patent Document 2).
However, when the light amount correction pattern is printed as means for improving the brightness uniformity, a part of light amount is blocked. Therefore, a utilization ratio of the light amount emitted from the cold-cathode tubes decreases, which leads to a problem in that sufficient brightness cannot be obtained. The use of the wavy reflection plate complicates a device configuration, thereby resulting in a problem in that the back-light device becomes complicated.
On the other hand, in the light diffusion plate used in the direct type back-light device, a material in which light diffusing agent is dispersed in a transparent resin is often used. However, in the light diffusion plate using such a material, when concentration of the light diffusing agent is increased to improve the brightness uniformity, there is a problem in that the brightness is decreased. Therefore, to solve this problem, there has been proposed the formation of a pattern such as a prism form or the like on a surface of the light diffusion plate to provide a diffusion effect exerted by the surface shape without decreasing the brightness (Patent Documents 3, 4, and 5). However, such a prismatic pattern on the surface of the light diffusion plate alone cannot achieve sufficient improvement in the brightness uniformity.
For example, in the Patent Documents 3 to 5, as a shape of prismatic ridges, a sawtooth cross-sectional shape is used. In addition thereto, instead of a sawtooth-shaped cross-sectional protrusion, there has been proposed the use of a sheet having a polypyramid-shaped protrusion to enhance a brightness improving effect (Patent Document 6). However, when the sheet having the polypyramid-shaped protrusion is used, the brightness uniformity cannot be improved.    [Patent Document 1] Japanese Patent Laid Open No. H6-273760 official gazette    [Patent Document 2] Japanese Patent Laid Open No. 2001-174813 official gazette    [Patent Document 3] Japanese Patent Laid Open No. H5-333333 official gazette    [Patent Document 4] Japanese Patent Laid Open No. H8-297202 official gazette    [Patent Document 5] Japanese Patent Laid Open No. 2000-182418 official gazette    [Patent Document 6] Japanese Patent No. 3134422 official gazette